Here I include images of selected values with dated postmarks. After studying all of the issues for this watermark, I find that the 30c is only 1E1, and the 25c is only 1E2. It is interesting that stamps overprinted mostly in 1936, and in a small number of cases in 1937, are found used in 1939/40, when the "servicio oficial" issues were already in use. I postulate that these stamps were infrecuently used by this ministry.

These two pairs have the same two plate varieties side by side, but the overprints are not identical. These plate varieties remind me of the alternating row pair of plate varieties in the type I 10c red plate. It is interesting to note that both pairs were postmarked on the same town and on the same day. They were likely from the same block.

The first pair:

left stamp:

right stamp:

The second pair:

left stamp:

right stamp:

Maybe someone has a large block of this stamp showing these repeating plate varieties?

Now that I have scanned a large number of these stamps with the ondulating rays watermark from 1935 to 1941, I aim to describe all of the early printings-those over the first two papers, 1E1, and 1E2.
Durante my brief foray into the M.A. Departmentals, I noticed that all 25c are from 1937 and 1E2, and all 30c are from 1936, and 1E1.

This is my plan towards an issue chronology during 1935/36/37:

1. The 1E1 "cents" issues from October 1, 1935 (1/2c, 1c, 2c, 3c SM green, 4c gray, 5c offset, 6c, 10c I typographed, 12c brown, 20c JMG)
2. The Departmentals for this group 1, all placed in circulation end of 1935 and beginning 1936.
3. The 1c, 5c, 10c II typographed, and the offset 15c dark blue cattle, 20c MG light blue, and the values between 25c and 20 pesos ( 1 peso only with boundaries) all on the 1E1 paper.
4. The Departmentals for this group 3, all placed in circulation throughout 1936 (5c typographed, 10c II, 15c, 20c MG, 25c, 30c, 50c, 1p without boundaries).
5. The 1 peso without boundaries 1E1 and 1E2, from February 1937 onwards. This stamp may be our best shot to define the issuing boundary between the 1E1 and the 1E2 paper.
6. The Departmentals for this group 5.
7. The 'cents' issues on the 1E2 paper, for which so far I only find the 1/2c?
8. The values from 25c to 20 pesos on the 1E2 paper, for which I do not find only the 2p, 10p, and 20p. It may be that these stamps do exist.
9. The Departmentals for this group 8 (on the 1E2 paper) for the 25c, 30c, 50c, 1p without boundaries.

An for each of these we need to determine which come with the watermark reading horizontally or vertically. Reminder of what the 1E2 looks like:

Horizontally:

Vertically:

A lot went on during the first two and a half years of issue for these stamps!

I hope I get some help with this work. If you use the 25c M.A.'s as guides, they are all 1E2. And if you use the 30c M.A., they are all 1E1.

About the 10c Rivadavia Red the articulo published by Tenorio Casal mentions the following:

During the first printings of this issue in 1935, "photoetched typography was used, which was used with success for the 10 centavos value. Te meet immediate needs, 1.200.000 stamps were printed, with the unique feature that the the design is more than 0,5 mm larger than the other values of the issue...At the same time there is a printing error, in which the tip of the inner coat as it meets the neck has a gap of missed design. After this quantity was used up, a second printing was placed in circulation December 17, 1935, with the smaller design. There are two types which can be differentiated in that in the first one the leftmost pocket near the name Bernardino is completely drawn in with color, while in the second one it is deffective."

"In september 1938 a third plate was used in two printings, with design dimensions similar to the second plate but with the leftmost pocket drawn in as in the first plate, and which features make differentiation of this stamp difficult."

"First plate: onlyu one printing with all the Ministry Officials.
Second plate: two printings with all the Ministry Officials, except M.R.C.
Third Plate: two printings with all the Ministry Officials, except M.A., M.J.I. and M.R.C. These were also overprinted 'Servicio Oficial'."

Here are representative images of the four selvages found on this stamp. Next is to match these conclusively to the plates as described by Tenorio. It seems that the plate 1 early low volume printing he refers to is the first selvage, which has a small red stripe along the edge:

The last selvage found on this stamp correspond to the most common printing. This stamp was printed in a continuous roll press, and the selvage stripes are vertical to avoid ink collecting on the plate selvage. This selvage is for Type II stamps.

I use my own reference scheme to describe Arg3551. This reference scheme enables me to have a more complete listing of the issues. I consider it a temporary scheme until I am able to arrive at a fairly complete listing.

The scheme combines:
1. The donomination in a computer-friendly format: For example, instead of 1/2c I use 05c, and instead of 2 1/2 c, I use 2p5c.
2. Mention of the person on the stamp (optional) or name acronym: I need this naming convention for the 3 centavos stamp, which was issued with the San Martin (SM) and Mariano Moreno (M) designs. The 20c Martin Guemes was issued with (JMG) or without (MG) the middle name shown.
3. Reference to the paper if I know of more than one: There are 30c1E1, 30c1E2, etc. If the stamp was only issued on one paper, there is no need for this naming convention, as is the case for the 3cSMGr, which was only issued on the 1E1 paper.
4. An additional reference for a specific plate: This naming convention is required for the 10c Rivadavia red, with types I and II, and the 10c Rivadavia Brown, with types A and B.
5. An additional reference for a scpecific color: This naming conventions is required for the 15c Small Format Cattle, issued in dark blue and only on the 1E1 paper, as 15cSC-D, and also issued in light blue and on a later paper, as 15cSC-L.

I mention several examples that show how my naming convention works:
1. The 8c was issued on one design, on one paper, on one plate, and on one color. Reference: 8c.
2. The 10c Rivadavia was issued in red and a range of browns, on many papers, and on at least four plates. Example references: 10cR-I, 10cBRCL1-A.

For the 18 papers I use the following scheme:
1. The early papers with the first watermark are the 1Ex papers, with x as of this edition being 1 to 4.
2. The clay papers were printed in two groups, CL1A and CL1B in 1943; and CL2A and CL2B in the 1950s.
3. The unwatermarked papers are of two types: grid from 1945 (NGR), and opaque from approximately 1948 (NOP).
4. The paper with the second watermark is found in three types: clear (2C), diffuse (2D), and with narrow rays (2N).
5. The late papers with the first watermark are the 1Lx papers, with x as of this edition being 1 to 5.

Thanks to a reference provided by one of the ‘Foreros,’ I learned about the existence of the book published by the Argentinean Post Office, Volume I, in 1939, by Antonio Deluca, and titled “Stamps and other postal and telegraph issues.” This book contains key information about Arg3551, about which Deluca mentions the following:

The decision to replace the San Martin issue by a new series came from 1931, but was abandoned due to the Argentinean Post Office ‘s economic hardship. Its director, Mr. Carlos Risso Dominguez, sent a memorandum to the Ministry of the Interior, dated November 28, 1932, in which he outlines basic facts about this series that I did not know before I obtained this book. The basic facts contained in this memorandum are:

1. There were several postal forgery incidents that cost the Argentinean Post Office a large loss of revenue. “In 1921 a postal forgery of the 5c stamp was found, and it incurred a loss of aproximately 1 million pesos of national currency in a few months. There seems to be an additional forgery of higher quality and affecting the 2c and 5c values. It is then without doubt that the prolonged use of the same stamp type conspires against its legitimacy and affects adversely our collection of revenue.”
2. Four categories were proposed for the new issue:

“a) Publish the likenesses of those signing the Declaration of Independence...”
“b) Publish the likenesses of those signing the 1853 Constitution...”
“c) Publish a selection of the likenesses of important military and civilian figures...and in addition add simbolic figures representing the Republic as shown on our currency, and mainly the Argentinean shield in its authentic model.”
“d) Finally...use the stamps for an increased awareness of our products and therefore put in effect a news-wrothy promotion in its favor, just as other countries do...”

There then take place several bureaucratic steps tipically required for a new stamp series: authorization by the Ministry of the Interior, design contest, and authorization by the President of the Republic. The second memorandum containing facts about this series was sent by the commission making recommendations on this new issue to the Argentinean Post Office on July 4 1933:
How this seriers came about

1. “ The commission proposes the portraits for the following important figures to be featured in as many issues: San Martin, Rivadavia, Moreno, Belgrano, Sarmiento, Mitre, Urquiza, Rodriguez, Guemes, Velez Sarsfield. Within the context of promoting, the commission indicates, of course, the map of the Argentinean Republic, and the following industries: Cattle, Agriculture, Oil, Wine-making, and Sugar Cane.”
2. This memorandum recommends the use of papel without watermark, somewhat thicker than the one being used at the time for typographed printing, and with white gum. It is interesting that the characteristics in this recommendation correspond to only one of the 18 papers for Arg3551: the NOP, or opaque paper not in the catalogs from aproximately 1948.
3. The recommended dimensions are: 19 by 24 mm, and 21 by 28 mm.
4. The designs and initial printing quantities recommended are:
1/2c Urquiza (50 millones); 1c Guemes (30 millones); 3c Rodriguez (120 millones); 5c Agricultura (60 millones); 6c Sarmiento (40 millones); 10c Belgrano (300 millones); 15c Mapa (20 millones); 20c Mitre (10 millones); 30c Sugar (12 millones); 35c Cattle (6 millones); 40c Wine-making (10 millones); 50c Velez Sarsfield (6 millones); 1p Oil Industry (2.5 millones); 5p Rivadavia (50000), 10p Moreno (20000), 20p San Martin (10000).
5. Only one design is recommended for the oficial issues, with each denomination having its own color: “The current system, is unappealing and very costly, because it forces specialized printings of the overprints. In addition, the wide range of papers and printings of the stamps and of the very same overprints, cause that collectors seek them, causing a disfunctional inventory, given that they cannot be acquired at post offices...” This memorandum includes other details about the official issues, including proposed values and printing quantities.

The Casa de Moneda (the Argentinean Treasury, in charge of printing stamps) makes the following design and respective denomination recommendations to the Argentinean Post Office on May 23, 1934:
Mitre 1/2c y 1c; Sarmiento 2c; Moreno 10c; Belgrano 5c y 20c; Southern National Park 12c; Sugar 10c; Argentinean Republic, wheat 15c; America and the Argentinean Republic, fruits of the country 5c; Oil 2c; Agriculture 10c; Republic and the farmer 5c; Christ of the Andes 2c; Republic and Shield 12c; Wheat Stalks 5c y 10c; Allegorical figure and wheat 10c; Iguazu Falls 50c. The most interesting fact in this memorandum is mention of Iguazu Falls. This memorandum mentions many designs that were not adopted.

Deluce mentions documents that relate to collaboration between the Argentinean Treasury and the Argentinean Post Office, it which the adopted characteristics are outlined: the use of a small format for the values up to 20c, and of the large format for values 25c and up.

On July 16 of 1934 the Patriot values as we know them from 1/2c to 20c were finalized. Durante the period spanning October 25, 1934 and February 13, 1935 the Resources values as we know them from 25c to 20 pesos were finalized.

On September 14, 1935, the Argentinean Post Office took the following actions:

1. Decides to issue on October 1, 1935 the 1/2c, 1c, 2c, 3c, 4c, 5c, 6c, 10c, 12c, y 20c (fullname version: JMG) values.
2. Demonetizes from January 1, 1936 onwards the previous (San Martin) issue.
3. Allows the exchange of San Martin stamps for the new stamps during the first 90 days of 1936.

According to Deluca, public notice of the new issue “was made by special announcements, and the printing of 5000 stamps for each value.” I speculate that these stamps are the ones we come across with specimen ( “MUESTRA”) overprint.

Here begins my first stab at grouping these stamps in a way that can help us identify all of the printings in this complex series.

This group is the first batch of "cents" issues. I am also including the 10c typographed. All of these stamps can be found on philatelic covers from the exhibition that took place in Buenos Aires in late October 1935.

The stamps are:
1/2c, 1c, 2c, 3c San Martin Green, 4c Gray, 5c offset, 6c, the various typographed printings of the 10c Rivadavia Red, the 12c Brown, and the two variations of the 20c Martin Guemes (Juan Martin Guemes-or JMG, and Martin Guemes-or MG). I am also including in this group the 20c Martin Guemes, even though it was issued sometime in 1936. This is a scheme in progress....

This group is for the stamps in group 1 overprinted with Ministerial official overprints. The base stamps are the 1c, 2c, 3c San Martin Green, 5c offset, 10c Rivadavia Red, and the 20cJMG. I am including the 20cMG in this group for now even though it was issued sometime in 1936.

I find that there are four variations of the 10c Rivadavia Red stamps as issued with these overprints: each type (I and II) was printed in dark colors and then in lighter colors from worn plates. The Type I was mostly printed in dark red, and I am guessing that there aren't many type I's in light red (I only have the M.M.). There is the need to look through a bunch of these stamps and sort out the ones that were actually issued.

This group corresponds to those stamps from group 3 that were issued as Ministerial overprints. The 25c was only issued on the 1E2 paper. The ones issued with this paper are the 15cSC-D, 30c, 50c, and 1pL.

Now that I have typed the first four groups, I plan to type all of the remaining groups up to 1944:

5. The 1p without boundaries 1E1 and 1E2.
6. The Departmental Officials and 'Servicio Oficial' issues for the 1p without boundaries.
7. The typographed issues of 1937 and onwards on the 1E1 paper: 1c, 5c, and 10c last plate.
8. The Departmental Officials and 'Servicio Oficial' issues for group 7.
9. The 1E2 issues. Because I only find the 1/2c for the small format issues, I am grouping the entire range here for this paper (1/2c to 20p).
10. The Departmental Officials and 'Servicio Oficial' issues for group 9.
11. The 1939 clay paper issues for the 5c and 10c Brown.
12. The 'Servicio Oficial' issues for group 11.
13. The 1939 color changes for the "cents" issues and all small format on the 1E3 paper.
14. The 'Servicio Oficial' issues for group 13.
15. The 25c to 20 pesos issues on the 1E3 paper.
16. The 'Servicio Oficial' issues for group 15.
17. The 15c Martin Guemes and the 20c Large Format cattle on the 1E3 paper from 1942, including the 15cMG 'Servicio Oficial.'
18. The 1942 issues on the 1E4 paper from 1/2c to 20 pesos.
19. The 'Servicio Oficial' issues for group 18.
20. The 1943 clay paper issues 5c/10c and large format stamps.
21. The 'Servicio Oficial' issues for group 20.
22. The second watermark issues from before 1945: at least I know about the 20cLC for these (2N paper).
23. Are there 'Servicio Oficial' issues for group 22?

Here are three specimens of the 5c1E with this variety. From the number of specimens that I have, and from small variations in the design of these three specimens, it it possible that there is more than one position on the plate with this variety. Does anyone have a block with more than one of these?

In addition, I do not find any 5c1Et (typographed) with this variety. I wonder if the error was corrected before the typographed stamps were overprinted.

There are three major printing types for this time period. The first printing, for which the earliest specimen I have is dated in June 1937, is light brown and printed on the 1E1 paper.

The second type is a red medium brown, and dates from early 1938. It is printed on the 1E1 paper and shows various stages of deep plate wear, especially on the face of Moreno.

The third type looks a lot like the second, except it is printed on the CL1A paper, the first clay paper. This paper is not as thin and shiny as the CL1B paper. An early specimen, from August 1938, is shown below.